The present invention relates to a highpass filter of a filter arrangement for a three-phase network connected to converters, having R, L and C sections for two harmonics.
From IEEE Transactions on Power Apparatus and Systems, vol. PAS 102, no. 9, September 1983, especially page 3209, a highpass for two characteristic 11th/13th-order harmonics is known.
For other than characteristic harmonics of adjacent order, additional tuned filter circuits or preferably, individual highpasses are often required for filtering harmonic currents at a high voltage level, which however, can often either not be operated or built economically in the necessary time for meeting the reactive power conditions, since the sum of-reactive power condition for the network cannot be met.
In addition, a point of parallel resonance occurs in the range of low-order harmonics of the second to about the fifth order between the network and the entire filter arrangement which, with a corresponding harmonic current, can result in excessive individual distortion in the voltage. However, excessive overvoltages can occur if the corresponding filter frequency coincides with the frequency of a harmonic. With low-order and low-power filters, the harmonic voltage can be limited to permissible values. For protection against additional stresses from the network, for instance, saturation and resonance phenomena when transformers are switched on, the filters must be overdesigned and optionally damped, increasing the losses. It is further a disadvantage that, as already mentioned, the required compensation power for low-order filter circuits for filtering the characteristic harmonics is not available.